Valve position control

ABSTRACT

A control system is provided for a turbine valve. The turbine valve has a first coil and a second coil to control or sense movement of a mechanical valve positioner. Two valve positioners are provided with each valve positioner having two drive circuits to drive the first and second coils. Switches are provided such that only one drive circuit is connected to each coil at a time. The control system may also include a hydraulic pilot valve section and a main hydraulic valve section. Feedbacks are used to determine a pilot valve error and a main valve error which are combined to determine a turbine valve error. The turbine valve error is repeatedly determined to minimize the error.

BACKGROUND

The present inventions relate generally to valves, and moreparticularly, to controlling the position of a valve.

Electrical power plants employ large steam turbines to generateelectricity. In a steam turbine, a main steam valve is used to controlthe flow rate of steam to the turbine. Thus, accurate valve position isimportant for efficient and safe operation of a steam turbine. Theinventions disclosed herein may also be used to control the position ofother valves as well.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

The invention may be more fully understood by reading the followingdescription in conjunction with the drawings, in which:

FIG. 1 is a schematic of a linear variable differential transformer(LVDT).

FIG. 2 is a schematic of drive circuits for redundant LVDTs.

FIG. 3 is a schematic of drive circuits for redundant servo positioningcoils.

FIG. 4 is a schematic for determining turbine valve error.

FIG. 5 is a schematic of a control system for a turbine valve.

FIG. 6 is a perspective view of a valve positioner module and a base.

FIG. 7 is a perspective view of the base.

FIG. 8 is a schematic of a hydraulic pilot valve section and a mainhydraulic valve section of a turbine valve.

DETAILED DESCRIPTION

One aspect of the preferred embodiments is the use of relays (SolidState Relays “SSRs”) that make/break connections to field I/O devices.For a redundant pair, only the module acting as the primary moduleactivates its relays, which allow the same field devices to be connectedto both modules. Preferably, redundant field devices are allowed toconcurrently connect to both (redundant) valve positioners.Additionally, a single termination unit may be used, thus eliminatingthe need for two termination units. By contrast, other valve positionersmay use separate devices for the individual modules in the redundantpair.

A LVDT (Linear Variable Differential Transformer) is shown in FIG. 1 .The LVDT measures the valve position as a feedback device. The LVDTincludes a rod that moves between a primary coil and two secondarycoils. The primary coil may be energized with an AC voltage at aspecific frequency specified by the manufacturer. As the positioning rodmoves in and out of the device, the amplitude of the secondary coilsvary linearly with the position of the rod. In a redundantconfiguration, one of the modules excites the primary coil by closingits relays to the device. The backup module opens its relays to avoidboth modules trying to drive the coil. The secondary coils can connectto both modules, so each measure the same feedback. Upon failover of theprimary module, the backup closes its relays as the primary opens itsrelay thus transferring the excitation of the primary coil to the backupmodule.

One feature of the preferred embodiments includes using the SSRs toisolate the drivers of the redundant modules. Another feature is theinter-module communication between the primary and the backup modules.Another feature is the ability of both modules to be connected to thesame devices, so they measure the same values.

A redundant LVDT primary oscillation diagram is shown in FIG. 2 . TheLVDT senses valve position. As shown, each valve positioner modulepreferably has two separate LVDT driver circuits to communicate with theredundant field devices (i.e., one driver circuit communicates with oneof the field devices and the other driver circuit communicates with theother field device). By using the SSRs in the LVDT driver circuits, thevalve positioner module may choose which field device to receive valveposition measurements from.

A redundant servo coil driver diagram is shown in FIG. 3 . The servocoil causes movement of the valve. Like FIG. 2 , each valve positionermodule preferably has two separate servo coil driver circuits tocommunicate with the redundant field devices. By using the SSRs in theservo coil circuits, the valve positioner module may choose which fielddevice to send valve position control signals to.

One improvement associated with the preferred embodiments includes theisolation of drivers to isolate the redundant modules. Anotherimprovement includes the ability to connect to the same I/O devices andrequiring only a single termination unit. Another improvement includesthe inter-module communication to accomplish fast and bumpless transfer.

A principal application for the preferred embodiments is generally largesteam turbines at electrical power plants. In this application, thecontrol system interfaces with main steam valves requiring redundantvalve positioners. Furthermore, other smaller machines may also requiresimilar redundancy, and the control system may be used in other systemsrequiring redundant valve controllers.

A problem overcome by the preferred embodiments is the elimination ofthe need to provide separate devices for the individual modules in aredundant pair. The preferred embodiments allow for the redundant fielddevices to concurrently connect to both (redundant) valve positioners.Moreover, a single termination unit may be connected to eliminate theneed for additional hardware.

One advantage of the preferred embodiments is that redundant fielddevices can concurrently connect to a redundant pair of valvepositioners. Another advantage is that the field device can connect to asingle termination unit. Another advantage is eliminating the need foradditional hardware. Another advantage is allowing for a nearly seamlesstransfer of coil drivers upon failover.

In the preferred embodiments, the module has redundant Solid StateRelays (SSRs) that interface with the redundant field devices.Initially, the primary module activates the SSRs connecting the moduleto the I/O, while the backup module deactivates its SSRs to the I/O. Theredundant pair of modules communicate with each other via acommunication bus. Each module provides the status of its hardware andoperating condition. The primary module informs the backup of theoperating condition so the backup can take over control upon failoverfrom the primary to the backup. Upon failover the primary deactivatesits SSRs while the backup activates its SSRs. This transfer may occur inless than 1 millisecond allowing for a nearly seamless transfer. Sincethe backup (now primary) has operating conditions from the other moduleit can take control immediately. Furthermore, each module informs thecontroller of its status, so the operator becomes aware of a failedmodule to allow time to replace the failed module. This can be doneon-line so there is no interruption of the operation of the system.

Preferably, the system includes two redundant valve positioners. Each ofthe valve positioners may be housed in a separate hardware module. Thevalve positioner modules may be plugged into one or more terminationunits, which provide an interface between the valve positioner modulesand the field devices. The termination unit may also be thought of as abackplane of the system. Preferably, both of the redundant field devicesare connected to both of the valve positioners. As described, tworedundant valve positioners are also preferably provided with each valvepositioner module including SSRs to control which valve positioner isconnected to the field devices. The field devices sense and/or controlthe valves.

Another aspect of the preferred embodiments is a cascaded PI(proportional integral) control loop that may be used for anon-mechanically linked servo valve. The control loop utilizes twofeedback devices to form a closed-loop control system. One of thefeedback devices reports the position of the pilot valve section of thevalve. The other feedback device reports the position of the mainhydraulic valve section of the valve. The two sections form a cascadedprocess.

Unlike the typical process control employed for a non-mechanicallylinked valve that utilizes an inner/outer loop strategy (cascadecontrol), the preferred embodiments resolve the errors of both parts ofthe process within a single control process. Moreover, the inner/outerloop strategy is typically implemented using separate processorsoperating on different modules, which often leads to difficulty ingetting the operation to settle without oscillation due to the time lagin communication between the two processors and the different transferrates of the two parts of the process. Preferably, the control of thepreferred embodiments resides wholly within a single processor, whicheliminates the problems caused by time lag that is inherent with theinner/outer loop strategy. Also, the preferred embodiments employ aweighting factor that is applied to the main valve error in order tocompensate for the difference in transfer ratios between the two partsof the process.

In order to hold the main hydraulic valve section steady, the pilotvalve must be held at its midpoint, referred to as the bias position.The value of the Pilot Valve Bias Position parameter sets the biasposition.

The difference between the bias position and the pilot valve positionyields an error value. The difference between the demand value and themain hydraulic valve position yields another error value. The two errorvalues are used to generate a current output to move the valve. Thecontrol algorithm ramps the output current up and down in order toproduce smooth movement of the valve. The control algorithm containselements that prevent the condition commonly referred to as “integralwindup” from having an impact on the control operation.

Because the two sections of this valve have different process transferrates, one of the valve errors is multiplied by the value of the WeightGain parameter to balance the control process. The error that has theweight gain applied to it is selected by the Weight Gain Applicationparameter.

The values of the Proportional Gain and Integral Factor parametersestablish the rate of movement, and how quickly the control loop settlesout.

The equation used in the PI algorithm has the form shown below, where Eis the position error, O is the coil drive output, and A₁ and A₀ arecoefficients derived from the values of P and I.O _(k+1) =A ₁ ·E _(k+1) +A ₀ ·E _(k) +O _(k)

The preferred embodiments use one of the equations shown below tocompute the position error, where E is the position error, D is the mainhydraulic valve position demand, R_(m) is the reported main hydraulicvalve position, B is the pilot valve bias position, R_(p) is thereported pilot valve position, and W is the value of the Weight Gainparameter. The equation that is used depends on the value of the WeightGain Application parameter.E=(D−R _(m))·W+(B−R _(p)), Weight Gain Application=0E=(D−R _(m))+(B−R _(p))·W, Weight Gain Application=1

The values of the coefficients are set by the equations shown below,where P is the proportional gain, I is the integral factor, and T_(s) isthe sample time.

${A_{1} = {{I \cdot \frac{T_{s}}{2}} + P}},{A_{0} = {{I \cdot \frac{T_{s}}{2}} - P}}$

To utilize a P only control loop, set the value of the Integral Factorparameter to 0. The value of A₀ may be set to 0 in the preferredembodiments. The equation used in the control loop may be reduced to theform shown below.O _(k+l) =P·E _(k+1) +O _(k)

The control algorithm will not allow the value of O to become greaterthan the maximum coil current, as selected by the DIP switch. When arequest is made for the preferred embodiments to move the valve awayfrom a stop position, the system clears the old output value to counterthe effects of integral windup, and thus there is no delay in valvemovement.

An exemplary control algorithm is shown in FIG. 4 .

A principal application for the preferred embodiments is generally largesteam turbines at electrical power plants. In this application, thecontrol system interfaces with a main steam valve controlled by a pilotvalve. Furthermore, the control system may also be used on other smallermachines.

A problem overcome by the preferred embodiments is the need to controlthe position of a servo valve actuator that utilizes a two phasecascaded control process, each with its own position indicatingtransducer and each with its own transfer function. Another problemovercome by the preferred embodiments is that control must be stable andfast responding, while providing for a user friendly and intuitivemethod to balance out the two control loops to achieve optimal control.The preferred embodiments also address the physical constraints andconfiguration requirements of various implementations of this type ofactuator with a single solution. The preferred embodiments also improveupon hardware solutions designed to control one specific implementationof this type of actuator. These solutions are specific to one actuatorand are not readily configurable or tunable for either the setting ofthe PID values or the compensation for the rate difference of thetransfer functions. The preferred embodiments also improve upon theutilization of two individual controllers to control the operation ofeach phase of the cascaded process separately, using an inner loop/outerloop interaction between them. These solutions can prove difficult toimplement due to the timing constraints built into the requiredinteraction between the two phases.

One advantage of the preferred embodiments is that soft configuration ofthe PID settings provides the flexibility to apply this solution tovarious implementations of this type of actuator. Another advantage isthat soft configuration of the PID settings provides the ability to tunethe operation of the control functionality. Another advantage is thatsoft configuration of the compensation for the rate difference of thetransfer functions provides the flexibility to apply this solution tovarious implementations of this type of actuator. Another advantage isthat the solution may be provided via a single module.

The preferred embodiments provide configurable parameters built into thesoftware control algorithm to provide on-the-fly adjustment of thecontrol configuration so that it may be used to control anyimplementation of this type of actuator. The solution provided by thepreferred embodiments may be contained within a single module, so thatit is not necessary to use multiple modules with tight timingcharacteristics between them.

The position of the pilot valve may be controlled by the bias demand.The position of the pilot valve controls oil flow to the main valve (orsteam valve) to control the position of the main valve. The positiondemand for the main valve is the desired set value for the main valve.Thus, in order to achieve the desired set value for the main valve, thebias demand must be controlled.

The inventions as described herein may have one or more of the followingfeatures in addition to any of the features described above. Referringto the figures, the following features are shown.

A system for redundant valve positioning including a mechanical valvepositioner 10 connected to a turbine valve 12 to open and close theturbine valve 12, the mechanical valve positioner 10 comprising a firstcoil 14A, 16A and a second coil 14B, 16B to control or sense movement ofthe mechanical valve positioner 10; a first valve positioner 18comprising a first drive circuit 20A, 22A and a second drive circuit20B, 22B, the first drive circuit 20A, 22A comprising a first driver 24,26 and a first switch 28, and the second drive circuit 20B, 22Bcomprising a second driver 24, 26 and a second switch 28, the firstdrive circuit 20A, 22A being in communication the first coil 14A, 16Aand the second drive circuit 20B, 22B being in communication with thesecond coil 14B, 16B; a second valve positioner 30 comprising a thirddrive circuit 20A, 22A and a fourth drive circuit 20B, 22B, the thirddrive circuit 20A, 22A comprising a third driver 24, 26 and a thirdswitch 28, and the fourth drive circuit 20B, 22B comprising a fourthdriver 24, 26 and a fourth switch 28, the third drive circuit 20A, 22Abeing in communication the first coil 14A, 16A and the fourth drivecircuit 20B, 22B being in communication with the second coil 14B, 16B;wherein only one of the first switch 28 and the third switch 28 isclosed at a time, the first driver 24, 26 and the third driver 24, 26thereby being alternately connected to the first coil 14A, 16A atdifferent times such that the first driver 24, 26 and the third driver24, 26 do not drive the first coil 14A, 16A at a same time; and whereinonly one of the second switch 28 and the fourth switch 28 is closed at atime, the second driver 24, 26 and the fourth driver 24, 26 therebybeing alternately connected to the second coil 14B, 16B at differenttimes such that the second driver 24, 26 and the fourth driver 24, 26 donot drive the second coil 14B, 16B at a same time.

The system for redundant valve positioning wherein the first coil 14A,16A and the second coil 14B, 16B are redundant coils.

The system for redundant valve positioning wherein the first coil 14Aand the second coil 14B are redundant position sensing coils 14A, 14B.

The system for redundant valve positioning wherein the first coil 14Aand the second coil 14B are each a primary coil 32 in a respective firstlinear variable differential transformer (LVDT) 34 and a second linearvariable differential transformer (LVDT) 34, the first LVDT 34 and thesecond LVDT 34 being redundant LVDTs.

The system for redundant valve positioning wherein the first LVDT 34comprises a first secondary coil 36 outputting a first position signal38 and the second LVDT 34 comprises a second secondary coil 36outputting a second position signal 38, the first secondary coil 36 andthe second secondary coil 36 both being in communication with the firstvalve positioner 18 and the second valve positioner 30 such that thefirst valve positioner 18 and the second valve positioner 30 bothreceive the first position signal 38 and the second valve positionsignal 38.

The system for redundant valve positioning wherein the communicationsbetween the first secondary coil 36, the second secondary coil 36, thefirst valve positioner 18 and the second valve positioner 30 arenon-switched such that the first valve positioner 18 and the secondvalve positioner 30 both receive the first position signal 38 and thesecond position signal 38 at a same time.

The system for redundant valve positioning wherein the first coil 16Aand the second coil 16B are redundant servo positioning coils 16A, 16B.

The system for redundant valve positioning wherein the first valvepositioner 18 and the second valve positioner 30 share a terminationunit 40 which interfaces with the first coil 14A, 16A and the secondcoil 14B, 16B.

The system for redundant valve positioning wherein a first field wire 42connects the first coil 14A, 16A to the termination unit 40 and a secondfield wire 42 connects the second coil 14B, 16B to the termination unit40, the first drive circuit 20A, 22A and the third drive circuit 20A,22A both communicating with the first coil 14A, 16A through the firstfield wire 42 and the second drive circuit 20B, 22B and the fourth drivecircuit 20B, 22B both communicating with the second coil 14B, 16Bthrough the second field wire 42.

The system for redundant valve positioning wherein the first valvepositioner 18 and the second valve positioner 30 are separate hardwaremodules.

The system for redundant valve positioning wherein the first valvepositioner 18 comprises a first circuit board 44 and the second valvepositioner 30 comprises a second circuit board 44, the first drivecircuit 20A, 22A and the second drive circuit 20B, 22B beingincorporated into the first circuit board 44, and the third drivecircuit 20A, 22A and the fourth drive circuit 20B, 22B beingincorporated into the second circuit board 44.

The system for redundant valve positioning further comprising a base 46with a first communications connector 48 and a second communicationsconnector 48, wherein the first valve positioner 18 is connected to thefirst communications connector 48 and the second valve positioner 30 isconnected to the second communications connector 48, the first valvepositioner 18 and the second valve positioner 30 communicating directlywith each other through the base 46 to control the first switch 28, thesecond switch 28, the third switch 28 and the fourth switch 28.

The system for redundant valve positioning wherein the base 46 furthercomprises a third communications connector 50, the first valvepositioner 18 and the second valve positioner 30 being in communicationwith a controller 52 through the third communications connector 50.

The system for redundant valve positioning wherein the first valvepositioner 18 is a primary module 18 and the second valve positioner 30is a backup module 30, the first switch 28 and the second switch 28being closed and the third switch 28 and the fourth switch 28 being openduring normal operation, and upon a failure of the first valvepositioner 18 at least the first switch 28 is opened and the thirdswitch 28 is closed to transfer control or sensing from the primarymodule 18 to the backup module 30.

The system for redundant valve positioning wherein the first switch 28,the second switch 28, the third switch 28 and the fourth switch 28 areeach a solid state relay 28.

The system for redundant valve positioning wherein the mechanical valvepositioner 10 further comprises a third coil 16A and a fourth coil 16B,the first coil 14A and the second coil 14B being redundant positionsensing coils, and the third coil 16A and the fourth coil 16B beingredundant servo positioning coils; the first valve positioner 18 furthercomprising a fifth drive circuit 22A and a sixth drive circuit 22B, thefifth drive circuit 22A comprising a fifth driver 26 and a fifth switch28, and the sixth drive circuit 22B comprising a sixth driver 26 and asixth switch 28, the fifth drive circuit 22A being in communication thethird coil 16A and the sixth drive circuit 22B being in communicationwith the fourth coil 16B; the second valve positioner 30 furthercomprising a seventh drive circuit 22A and an eighth drive circuit 22B,the seventh drive circuit 22A comprising a seventh driver 26 and aseventh switch 28, and the eighth drive circuit 22B comprising an eighthdriver 26 and an eighth switch 28, the seventh drive circuit 22A beingin communication the third coil 16A and the eighth drive circuit 22Bbeing in communication with the fourth coil 16B; wherein only one of thefifth switch 28 and the seventh switch 28 is closed at a time, the fifthdriver 26 and the seventh driver 26 thereby being alternately connectedto the third coil 16A at different times such that the fifth driver 26and the seventh driver 26 do not drive the third coil 16A at a sametime; and wherein only one of the sixth switch 28 and the eighth switch28 is closed at a time, the sixth driver 26 and the eighth driver 26thereby being alternately connected to the fourth coil 16B at differenttimes such that the sixth driver 26 and the eighth driver 26 do notdrive the fourth coil 16B at a same time.

The system for redundant valve positioning wherein the first coil 14Aand the second coil 14B are each a primary coil 32 in a respective firstlinear variable differential transformer (LVDT) 34 and a second linearvariable differential transformer (LVDT) 34, the first LVDT 34 and thesecond LVDT 34 being redundant LVDTs, the first LVDT 34 comprises afirst secondary coil 36 outputting a first position signal 38 and thesecond LVDT 34 comprises a second secondary coil 36 outputting a secondposition signal 38, the first secondary coil 36 and the second secondarycoil 36 both being in communication with the first valve positioner 18and the second valve positioner 30 such that the first valve positioner18 and the second valve positioner 30 both receive the first positionsignal 38 and the second valve position signal 38, and thecommunications between the first secondary coil 36, the second secondarycoil 36, the first valve positioner 18 and the second valve positioner30 are non-switched such that the first valve positioner 18 and thesecond valve positioner 30 both receive the first position signal 38 andthe second position signal 38 at a same time.

The system for redundant valve positioning wherein the first valvepositioner 18 and the second valve positioner 30 share a terminationunit 40 which interfaces with the first coil 14A, the second coil 14B,the third coil 16A and the fourth coil 16B, and a first field wire 42connects the first coil 14A to the termination unit 40, a second fieldwire 42 connects the second coil 14B to the termination unit 40, a thirdfield wire 42 connects the third coil 16A to the termination unit 40 anda fourth field wire 42 connects the fourth coil 16B to the terminationunit 40, the first drive circuit 20A and the third drive circuit 20Aboth communicating with the first coil 14A through the first field wire42, the second drive circuit 20B and the fourth drive circuit 20B bothcommunicating with the second coil 14B through the second field wire 42,the fifth drive circuit 22A and the seventh drive circuit 22A bothcommunicating with the third coil 16A through the third field wire 42and the sixth drive circuit 22B and the eighth drive circuit 22B bothcommunicating with the fourth coil 16B through the fourth field wire 42.

The system for redundant valve positioning wherein the first valvepositioner 18 and the second valve positioner 30 are separate hardwaremodules, the first valve positioner 18 comprises a first circuit board44 and the second valve positioner 30 comprises a second circuit board44, the first drive circuit 20A, the second drive circuit 20B, the fifthdrive circuit 22A and the sixth drive circuit 22B being incorporatedinto the first circuit board 44, and the third drive circuit 20A, thefourth drive circuit 20B, the seventh drive circuit 22A and the eighthdrive circuit 22B being incorporated into the second circuit board 44,further comprising a base 46 with a first communications connector 48and a second communications connector 48, wherein the first valvepositioner 18 is connected to the first communications connector 48 andthe second valve positioner 30 is connected to the second communicationsconnector 48, the first valve positioner 18 and the second valvepositioner 30 communicating directly with each other through the base 46to control the first switch 28, the second switch 28, the third switch28, the fourth switch 28, the fifth switch 28, the sixth switch 28, theseventh switch 28 and the eighth switch 28, and wherein the base 46further comprises a third communications connector 50, the first valvepositioner 18 and the second valve positioner 30 being in communicationwith a controller 52 through the third communications connector 50.

A system for controlling a turbine valve 60 including a hydraulic pilotvalve section 62 being moveable in a first direction 64 from a middleposition 66 and moveable in a second direction 68 from the middleposition 66 opposite from the first direction 64; a main hydraulic valvesection 70 being moveable in a first direction 72 to close the turbinevalve 60 when the hydraulic pilot valve section 62 is moved in the firstdirection 64 and moveable in a second direction 74 to open the turbinevalve 60 when the hydraulic pilot valve section 62 is moved in thesecond direction 68; a position demand 76 indicating a desired positionof the turbine valve 60; a first feedback 78 indicating an actualposition of the hydraulic pilot valve section 62; a second feedback 80indicating an actual position of the main hydraulic valve section 70; apilot valve error 82 determined by a difference 84 between the middleposition 66 of the hydraulic pilot valve section 62 and the firstfeedback 78; a main valve error 86 determined by a difference 88 betweenthe position demand 76 and the second feedback 80; a turbine valve error88 determined by combining 90 the pilot valve error 82 and the mainvalve error 86; and a pilot valve adjustment 92 moving the hydraulicpilot valve section 62 in response to the turbine valve error 88;wherein the turbine valve error 88 is repeatedly determined and thepilot valve adjustment 92 repeatedly moves the hydraulic pilot valvesection 62 to minimize the turbine valve error 88.

The system for controlling a turbine valve wherein the turbine valve 60is a steam valve 60.

The system for controlling a turbine valve further comprising a weightgain 94 applied to the pilot valve error 82 or the main valve error 86.

The system for controlling a turbine valve wherein the weight gain 94applies a greater weight to the pilot valve error 82.

The system for controlling a turbine valve wherein the turbine valveerror 88 further incorporates a proportional gain 96 and an integralfactor 98.

It is understood that the preferred embodiments described herein may beimplemented as computerized methods in a non-transitory computerreadable medium if desired.

While preferred embodiments of the inventions have been described, itshould be understood that the inventions are not so limited, andmodifications may be made without departing from the inventions herein.While each embodiment described herein may refer only to certainfeatures and may not specifically refer to every feature described withrespect to other embodiments, it should be recognized that the featuresdescribed herein are interchangeable unless described otherwise, evenwhere no reference is made to a specific feature. It should also beunderstood that the advantages described above are not necessarily theonly advantages of the inventions, and it is not necessarily expectedthat all of the described advantages will be achieved with everyembodiment of the inventions. The scope of the inventions is defined bythe appended claims, and all devices and methods that come within themeaning of the claims, either literally or by equivalence, are intendedto be embraced therein.

The invention claimed is:
 1. A system for redundant valve positioning,comprising: a mechanical valve positioner connected to a turbine valveto open and close the turbine valve, the mechanical valve positionercomprising a first coil and a second coil to control or sense movementof the mechanical valve positioner; a first valve positioner comprisinga first drive circuit and a second drive circuit, the first drivecircuit comprising a first driver and a first switch, and the seconddrive circuit comprising a second driver and a second switch, the firstdrive circuit being in communication the first coil and the second drivecircuit being in communication with the second coil; a second valvepositioner comprising a third drive circuit and a fourth drive circuit,the third drive circuit comprising a third driver and a third switch,and the fourth drive circuit comprising a fourth driver and a fourthswitch, the third drive circuit being in communication the first coiland the fourth drive circuit being in communication with the secondcoil; wherein only one of the first switch and the third switch isclosed at a time, the first driver and the third driver thereby beingalternately connected to the first coil at different times such that thefirst driver and the third driver do not drive the first coil at a sametime; and wherein only one of the second switch and the fourth switch isclosed at a time, the second driver and the fourth driver thereby beingalternately connected to the second coil at different times such thatthe second driver and the fourth driver do not drive the second coil ata same time.
 2. The system for redundant valve positioning according toclaim 1, wherein the first coil and the second coil are redundant coils.3. The system for redundant valve positioning according to claim 1,wherein the first coil and the second coil are redundant positionsensing coils.
 4. The system for redundant valve positioning accordingto claim 3, wherein the first coil and the second coil are each aprimary coil in a respective first linear variable differentialtransformer (LVDT) and a second respective linear variable differentialtransformer (LVDT), the first LVDT and the second LVDT being redundantLVDTs.
 5. The system for redundant valve positioning according to claim4, wherein the first LVDT comprises a first secondary coil outputting afirst position signal and the second LVDT comprises a second secondarycoil outputting a second position signal, the first secondary coil andthe second secondary coil both being in communication with the firstvalve positioner and the second valve positioner such that the firstvalve positioner and the second valve positioner both receive the firstposition signal and the second valve position signal.
 6. The system forredundant valve positioning according to claim 5, wherein thecommunications between the first secondary coil, the second secondarycoil, the first valve positioner and the second valve positioner arenon-switched such that the first valve positioner and the second valvepositioner both receive the first position signal and the secondposition signal at a same time.
 7. The system for redundant valvepositioning according to claim 1, wherein the first coil and the secondcoil are redundant servo positioning coils.
 8. The system for redundantvalve positioning according to claim 1, wherein the first valvepositioner and the second valve positioner share a termination unitwhich interfaces with the first coil and the second coil.
 9. The systemfor redundant valve positioning according to claim 8, wherein a firstfield wire connects the first coil to the termination unit and a secondfield wire connects the second coil to the termination unit, the firstdrive circuit and the third drive circuit both communicating with thefirst coil through the first field wire and the second drive circuit andthe fourth drive circuit both communicating with the second coil throughthe second field wire.
 10. The system for redundant valve positioningaccording to claim 1, wherein the first valve positioner and the secondvalve positioner are separate hardware modules.
 11. The system forredundant valve positioning according to claim 10, wherein the firstvalve positioner comprises a first circuit board and the second valvepositioner comprises a second circuit board, the first drive circuit andthe second drive circuit being incorporated into the first circuitboard, and the third drive circuit and the fourth drive circuit beingincorporated into the second circuit board.
 12. The system for redundantvalve positioning according to claim 11, further comprising a base witha first communications connector and a second communications connector,wherein the first valve positioner is connected to the firstcommunications connector and the second valve positioner is connected tothe second communications connector, the first valve positioner and thesecond valve positioner communicating directly with each other throughthe base to control the first switch, the second switch, the thirdswitch and the fourth switch.
 13. The system for redundant valvepositioning according to claim 12, wherein the base further comprises athird communications connector, the first valve positioner and thesecond valve positioner being in communication with a controller throughthe third communications connector.
 14. The system for redundant valvepositioning according to claim 1, wherein the first valve positioner isa primary module and the second valve positioner is a backup module, thefirst switch and the second switch being closed and the third switch andthe fourth switch being open during normal operation, and upon a failureof the first valve positioner at least the first switch is opened andthe third switch is closed to transfer control or sensing from theprimary module to the backup module.
 15. The system for redundant valvepositioning according to claim 1, wherein the first switch, the secondswitch, the third switch and the fourth switch are each a solid staterelay.
 16. The system for redundant valve positioning according to claim1, wherein: the mechanical valve positioner further comprises a thirdcoil and a fourth coil, the first coil and the second coil beingredundant position sensing coils, and the third coil and the fourth coilbeing redundant servo positioning coils; the first valve positionerfurther comprising a fifth drive circuit and a sixth drive circuit, thefifth drive circuit comprising a fifth driver and a fifth switch, andthe sixth drive circuit comprising a sixth driver and a sixth switch,the fifth drive circuit being in communication the third coil and thesixth drive circuit being in communication with the fourth coil; thesecond valve positioner further comprising a seventh drive circuit andan eighth drive circuit, the seventh drive circuit comprising a seventhdriver and a seventh switch, and the eighth drive circuit comprising aneighth driver and an eighth switch, the seventh drive circuit being incommunication the third coil and the eighth drive circuit being incommunication with the fourth coil; wherein only one of the fifth switchand the seventh switch is closed at a time, the fifth driver and theseventh driver thereby being alternately connected to the third coil atdifferent times such that the fifth driver and the seventh driver do notdrive the third coil at a same time; and wherein only one of the sixthswitch and the eighth switch is closed at a time, the sixth driver andthe eighth driver thereby being alternately connected to the fourth coilat different times such that the sixth driver and the eighth driver donot drive the fourth coil at a same time.
 17. The system for redundantvalve positioning according to claim 16, wherein the first coil and thesecond coil are each a primary coil in a respective first linearvariable differential transformer (LVDT) and a second respective linearvariable differential transformer (LVDT), the first LVDT and the secondLVDT being redundant LVDTs, the first LVDT comprises a first secondarycoil outputting a first position signal and the second LVDT comprises asecond secondary coil outputting a second position signal, the firstsecondary coil and the second secondary coil both being in communicationwith the first valve positioner and the second valve positioner suchthat the first valve positioner and the second valve positioner bothreceive the first position signal and the second valve position signal,and the communications between the first secondary coil, the secondsecondary coil, the first valve positioner and the second valvepositioner are non-switched such that the first valve positioner and thesecond valve positioner both receive the first position signal and thesecond position signal at a same time.
 18. The system for redundantvalve positioning according to claim 17, wherein the first valvepositioner and the second valve positioner share a termination unitwhich interfaces with the first coil, the second coil, the third coiland the fourth coil, and a first field wire connects the first coil tothe termination unit, a second field wire connects the second coil tothe termination unit, a third field wire connects the third coil to thetermination unit and a fourth field wire connects the fourth coil to thetermination unit, the first drive circuit and the third drive circuitboth communicating with the first coil through the first field wire, thesecond drive circuit and the fourth drive circuit both communicatingwith the second coil through the second field wire, the fifth drivecircuit and the seventh drive circuit both communicating with the thirdcoil through the third field wire and the sixth drive circuit and theeighth drive circuit both communicating with the fourth coil through thefourth field wire.
 19. The system for redundant valve positioningaccording to claim 18, wherein the first valve positioner and the secondvalve positioner are separate hardware modules, the first valvepositioner comprises a first circuit board and the second valvepositioner comprises a second circuit board, the first drive circuit,the second drive circuit, the fifth drive circuit and the sixth drivecircuit being incorporated into the first circuit board, and the thirddrive circuit, the fourth drive circuit, the seventh drive circuit andthe eighth drive circuit being incorporated into the second circuitboard, further comprising a base with a first communications connectorand a second communications connector, wherein the first valvepositioner is connected to the first communications connector and thesecond valve positioner is connected to the second communicationsconnector, the first valve positioner and the second valve positionercommunicating directly with each other through the base to control thefirst switch, the second switch, the third switch, the fourth switch,the fifth switch, the sixth switch, the seventh switch and the eighthswitch, and wherein the base further comprises a third communicationsconnector, the first valve positioner and the second valve positionerbeing in communication with a controller through the thirdcommunications connector.